1. Field of the Invention
The present invention generally relates to the drilling of subterranean wells. More particularly, the invention relates to an apparatus for sealing a wellbore during the formation drilling process. The invention further relates to a method of underbalanced drilling, in which the wellbore is selectively sealed during drilling in order to remove dill pipe and attached tools.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. The process of drilling typically includes the circulation of drilling fluids through the drill string. The fluids are pumped under pressure through the drill string and out ports disposed in or near the drill bit. The fluids are then circulated back to the surface on the outside of the drill string but within the formed wellbore.
The use of drilling fluid has multiple purposes. Drilling fluids serve to cool and lubricate the drill bit as it chews the rock formation en route to total depth. The fluids also permit cuttings from the formation to be lifted to the surface, thereby preserving the interface between the drill bit and the bottom of the formation. Most importantly, drilling fluids aid in controlling wellbore pressures by applying a hydrostatic force downward against the formation. This, in turn, prevents the formation from expelling formation fluids from the wellbore at a high pressure should the drill bit penetrate a high pressure zone.
Historically, drilling fluids have been weighted with tertiary material known as “mud.” Drilling mud increases the downward pressure. The weighting of fluid prevents the well from “kicking” or even causing a “blow out.” In an ideal situation, the mud is weighted so as to precisely counterbalance any upward force generated by formation pressures. However, because it is difficult to predict formation pressures in a timely manner, drilling operators will increase the weight of mud to an overbalanced state. This increases safety on the rig and prevents damage to the drilling equipment from a blow out.
There are disadvantages to overbalanced drilling. Primarily, the weight of drilling mud has been known to overcome the formation pressure to such an extent that the formation begins to receive the drilling mud. In this instance, drilling mud is lost to the formation and cannot be recirculated at the surface. This, in turn, requires that additional drilling mud be pumped downhole at great expense. Pumping cannot be discontinued or the well may ultimately lose all drilling fluids, causing the well to be in a dangerously underbalanced condition. Accordingly, drilling companies have recently explored ways of drilling formations in a controllably underbalanced state.
An underbalanced condition is one in which fluid pressure in a wellbore is less than fluid pressure in a formation intersected by the wellbore. There are several recognized advantages to drilling and completing a well in an underbalanced condition. First, underbalanced drilling helps prevent fluid loss from the wellbore into the formation. Those of ordinary skill in the art will appreciate that drilling mud is very expensive. Further, the loss of drilling mud into the formation can result in damage to the formation caused by infiltration of the drilling mud into the adjoining rock. Related to this, a clean formation, i.e., one without mud infiltration, allows for a better performing well and more accurate logging measurements of the well contents. An overview of underbalanced completion practices and their advantages may be found in an article entitled “Underbalanced Completions Improve Well Safety and Productivity” by Tim Walker and Mark Hopmann (World Oil, November, 1995), which is incorporated herein by this reference.
In some cases, oil and gas can be recovered during an underbalanced drilling process. The hydrocarbons supplement the drilling fluid. In some instances, the recovery of oil/gas from the well during underbalanced drilling has been sufficient to pay for the cost of drilling the well even prior to completion of the well. For a fuller discussion of advantages of underbalanced drilling, including methods of controlling the well using an exemplary rotating blow out preventer, please refer to U.S. Pat. No. 6,129,152, entitled Rotating BOP and Method, issued Oct. 10, 2000, 1998, to Hosie et al, which is incorporated herein by reference.
Underbalanced drilling creates certain challenges to the rig operator. One such challenge relates to the process of tripping the drill string out of the wellbore. In this respect, it is necessary from time to time to replace the drill bit or change out other downhole tools. It is also necessary to periodically stop the drilling process so that a string of casing can be run into a drilled section of the well and then cemented. A problem is encountered, however, when the drill string is being pulled from an underbalanced well. In this regard, the weight of the pipe becomes less than the upward pressure being exerted by the formation. This condition, known as “pipe light,” may occur when the length of the pipe becomes less than 1,500 to 1,000 feet. As the drill string becomes shorter, a danger grows that the formation may violently expel not only fluids from the formation, but the shortened drill string as well. In other words, formation pressure can actually push or accelerate the drill string out of the wellbore. In some instances, the blow out preventers may not be able to stop the upward movement of the pipe. Once the pipe string is moving upwardly, closing the rams may result in tearing the rams out rather than stopping the upward movement of the pipe. In this case, the rams will not be available to shut in the well after the pipe has been pushed from the wellbore, assuming there is someone left at the rig site to activate the rams after the drill pipe is ejected from the well. The forces are great enough so that ejected drill pipe may be found quite far from the rig. As well, sparks produced can ignite gas to produce a hot fire that can melt a drilling rig within minutes.
One method used to avoid a blow out situation is to kill the well prior to removal of the drilling string. Once the drill string is lowered back into the wellbore below the string light point, it may be possible to adjust the drilling fluids so that underbalanced drilling continues. However, formation damage may have already occurred that is substantially irreversible, and the advantages of underbalanced drilling may have been lost.
Another practice is that of providing a snubbing unit for removing the drilling string. However, the snubbing unit takes considerable time to rig up, requires considerable additional time while tripping the well, and then requires considerable additional time to rig down. Thus, the cost of tripping the drill string can be quite considerable due to the rig time costs and snubbing unit costs. Additional tripping of the well may also be necessary, and again require the snubbing unit. This procedure then, while effective and safe, increases drilling costs considerably.
Consequently, an improved apparatus and method is desired to aid in the removal of drill string from a wellbore that is drilled in an underbalanced state. Such an improved apparatus and method should enable the quick and safe removal of the drill string from the well without the need to kill the well. The apparatus and method should be useful for repeated tripping of the drill string whenever necessary without significant time and cost increases, and without need of a costly snubbing unit.
Further, a need exists for a well control tool that allows the well to be selectively shut in. In addition, a need exists for such an apparatus that may be attached to a drill string, production tubing string, or other tubular. In this manner, the apparatus may isolate a formation intersected by a wellbore in an underbalanced condition from the remainder of the wellbore while the tubular string is tripped in or out of the wellbore.
A wellbore isolation apparatus is also needed during a sidetrack drilling operation. A sidetrack drilling operation is conducted in order to create a lateral wellbore at a selected depth off of a primary wellbore. For the same reasons outlined above, it is desirable to drill lateral wellbores in an underbalanced state as well. Thus, a need exists for a well control tool that allows the primary wellbore to be selectively shut in during a sidetrack drilling operation above the depth of the lateral wellbore. In addition, a need exists for a diverter tool, such as a whipstock, that can be selectively raised above the depth of the lateral wellbore in order to seal off the lateral wellbore while the working string is tripped in and out of the primary wellbore.